Methods and apparatuses for cardiac arrhythmia classification using dynamic beatdriven morphological feature extraction

ABSTRACT

An implantable cardioverter/defibrillator (ICD) includes a tachyarrhythmia detection and classification system that classifies tachyarrhythmias based on a morphological analysis of arrhythmic waveforms and a template waveform. Correlation coefficients each computed between morphological features of an arrhythmic waveform and morphological features of the template waveform provide for the basis for classifying the tachyarrhythmia. In one embodiment, morphological features are collected from a sensed arrhythmic waveform, and temporally corresponding morphological features are extracted a stored template waveform.

CLAIM OF PRIORITY

This application is a continuation of U.S. patent application Ser. No.12/206,198, filed Sep. 8, 2008, which is a continuation of U.S. patentapplication Ser. No. 11/038,996, now issued as U.S. Pat. No. 7,430,446,filed Jan. 20, 2005, which are hereby incorporated by reference in theirentirety.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to co-pending, commonly assigned U.S. Pat.No. 7,039,463, entitled “DISCRIMINATION OF SUPRAVENTRICULAR TACHYCARDIAAND VENTRICULAR TACHYCARDIA EVENTS,” filed on Dec. 9, 2003, U.S. Pat.No. 7,031,764, entitled “CARDIAC RHYTHM MANAGEMENT SYSTEMS AND METHODSUSING MULTIPLE MORPHOLOGY TEMPLATES FOR DISCRIMINATING BETWEEN RHYTHMS,”filed on Nov. 8, 2002, U.S. Pat. No, 6,959,212, entitled “SYSTEM ANDMETHOD FOR ARRHYTHMIA DISCRIMINATION,” filed on Oct. 22, 2001, and U.S.Pat. No. 6,996,434, entitled “METHOD AND SYSTEM FOR VERIFYING THEINTEGRITY OF NORMAL SINUS RHYTHM TEMPLATES,” filed Aug. 2, 2001, whichare hereby incorporated by reference in their entirety.

TECHNICAL FIELD

This document relates generally to cardiac rhythm management (CRM)systems and particularly, but not by way of limitation, to such a systemproviding for morphology-based classification of tachyarrhythmias.

BACKGROUND

The heart is the center of a person's circulatory system. The leftportions of the heart, including the left atrium (LA) and left ventricle(LV), draw oxygenated blood from the lungs and pump it to the organs ofthe body to provide the organs with their metabolic needs for oxygen.The right portions of the heart, including the right atrium (RA) andright ventricle (RV), draw deoxygenated blood from the body organs andpump it to the lungs where the blood gets oxygenated. These mechanicalpumping functions are accomplished by contractions of the heart. In anormal heart, the sinoatrial (SA) node, the heart's natural pacemaker,generates electrical impulses, called action potentials, that propagatethrough an electrical conduction system to various regions of the heartto cause the muscular tissues of these regions to depolarize andcontract. The electrical conduction system includes, in the order bywhich the electrical impulses travel in a normal heart, internodalpathways between the SA node and the atrioventricular (AV) node, the AVnode, the His bundle, and the Purkinje system including the right bundlebranch (RBB, which conducts the electrical impulses to the RV) and theleft bundle branch (LBB, which conducts the electrical impulses to theIN). More generally, the electrical impulses travel through an AVconduction pathway to cause the atria, and then the ventricles, tocontract.

Tachyarrhythmia (also referred to as tachyarrhythmia) occurs when theheart contracts at a rate higher than a normal heart rate.Tachyarrhythmia generally includes ventricular tachyarrhythmia (VT) andsupraventricular tachyarrhythmia (SVT). VT occurs, for example, when apathological conduction loop formed in the ventricles through whichelectrical impulses travel circularly within the ventricles, or when apathologically formed electrical focus generates electrical impulsesfrom the ventricles. SVT includes physiologic sinus tachyarrhythmia andpathologic SVTs. The physiologic sinus tachyarrhythmia occurs when theSA node generates the electrical impulses at a particularly high rate. Apathologic SVT occurs, for example, when a pathologic conduction loopforms in an atrium. Fibrillation occurs when the heart contracts at atachyarrhythmia rate with an irregular rhythm. Ventricular fibrillationCVF), as a ventricular arrhythmia with an irregular conduction, is alife threatening condition requiring immediate medical treatment such asventricular defibrillation. Atrial fibrillation (AT), as a SVT with anirregular rhythm, though not directly life threatening, also needsmedical treatment such as atrial defibrillation to restore a normalcardiac function and to prevent the deterioration of the heart.Implantable cardioverter/defibrillators (ICDs) are used to treattachyarrhythmias, including fibrillation. To deliver an effectivecardioversion/defibrillation therapy, the cardioversion/defibrillationenergy is to be delivered to the chambers of the heart where thetachyarrhythmia or fibrillation originates. When the atrial rate ofdepolarizations (or contractions) is substantially different from theventricular rate of depolarizations (or contractions), the atrial andventricular rates of depolarizations (or contractions) provide for abasis for locating where the tachyarrhythmia originates. However, thereis a need to locate where the tachyarrhythmia originates when the atrialdepolarizations and the ventricular depolarizations present a one-to-one(1:1) relationship.

SUMMARY

An implantable cardioverter/defibrillator (ICD) includes atachyarrhythmia detection and classification system that classifiestachyarrhythmias based on a morphological analysis of arrhythmicwaveforms and a template waveform. Correlation coefficients eachcomputed between morphological features of an arrhythmic waveform andmorphological features of the template waveform provide for the basisfor classifying the tachyarrhythmia.

In one embodiment, a system for classifying tachyarrhythmias includes atemplate generation circuit, a template waveform storage circuit, afeature locating circuit, and a feature extracting circuit. The templategeneration circuit produces a template waveform associated with atemplate heart beat of a known type cardiac rhythm. The templatewaveform is stored in the template waveform storage circuit. The featurelocating circuit selects a plurality of arrhythmic morphologicalfeatures on an arrhythmic waveform associated with an arrhythmic heartbeat of a tachyarrhythmia and produces timing information indicative oflocations of the arrhythmic morphological features on the arrhythmicwaveform. The feature extracting circuit locates corresponding templatemorphological features on the stored template waveform based on thattiming information.

In one embodiment, a method for extracting morphological features fortachyarrhythmia classification is provided. A template waveformassociated with a template heart beat is produced and stored. Anarrhythmic waveform associated with an arrhythmic heart beat isreceived. A plurality of arrhythmic morphological features is selectedon the arrhythmic waveform. Timing information is produced to indicatelocations of the arrhythmic morphological features on the arrhythmicwaveform. Corresponding template morphological features are then locatedon the stored template waveform based on that timing information.

This Summary is an overview of some of the teachings of the presentapplication and not intended to be an exclusive or exhaustive treatmentof the present subject matter. Further details about the present subjectmatter are found in the detailed description and appended claims. Otheraspects of the invention will be apparent to persons skilled in the artupon reading and understanding the following detailed description andviewing the drawings that form a part thereof, each of which are not tobe taken in a limiting sense. The scope of the present invention isdefined by the appended claims and their equivalents.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings, which are not necessarily drawn to scale, illustrategenerally, by way of example, but not by way of limitation, variousembodiments discussed in the present document.

FIG. 1 is an illustration of one embodiment of a CRM system and portionsof the environment in which CRM system operates,

FIG. 2 is a block diagram illustrating an embodiment of atachyarrhythmia detection and classification system being part of theCRM system.

FIG. 3 is a flow chart illustrating an embodiment of a method formorphology-based 1:1 tachyarrhythmia discrimination.

FIG. 4 is a block diagram illustrating an embodiment of amorphology-based 1:1 tachyarrhythmia discrimination circuit being partof the tachyarrhythmia detection and classification system.

FIG. 5 is a block diagram illustrating an embodiment of a featureextractor being part of the morphology-based 1:1 tachyarrhythmiadiscrimination circuit.

FIG. 6 is a block diagram illustrating a specific embodiment of thefeature extractor of FIG. 5.

FIG. 7 is a flow chart illustrating an embodiment of a method forextracting morphological features for the morphology-based 1:1tachyarrhythmia discrimination.

FIG. 8 is a graph illustrating feature extraction using the method ofFIG. 7.

FIG. 9 is a flow chart illustrating a specific embodiment of the methodfor morphology-based 1:1 tachyarrhythmia discrimination as illustratedin FIG. 3 including an exemplary application of the method of FIG. 7.

FIG. 10 is a block diagram illustrating an embodiment of a correlationanalyzer and a 1:1 tachyarrhythmia classifier being part of themorphology-based 1:1 tachyarrhythmia discrimination circuit.

FIG. 11 is a block diagram illustrating a specific embodiment of thecorrelation analyzer and the 1:1 tachyarrhythmia classifier of FIG. 10.

FIG. 12 is a flow chart illustrating an embodiment of a method foranalyzing correlation and classifying 1:1 tachyarrhythmias for themorphology-based 1:1 tachyarrhythmia discrimination.

FIG. 13 is a graph illustrating an exemplary template band as used inthe method of FIG. 12.

FIGS. 14A-D include graphs illustrating a fuzzy decisional process inclassifying 1:1 tachyarrhythmias for the method of FIG. 12.

FIG. 15 is a flow chart illustrating a specific embodiment of the methodfor morphology-based 1:1 tachyarrhythmia discrimination as illustratedin FIG. 3 including an exemplary application of the method of FIG. 12.

FIG. 16 is a block diagram illustrating an embodiment of anothercorrelation analyzer being part of the morphology-based 1:1tachyarrhythmia discrimination circuit.

FIG. 17 is a block diagram illustrating a specific embodiment of thecorrelation analyzer of FIG. 16.

FIG. 18 is a flow chart illustrating an embodiment of another method foranalyzing correlation for the morphology-based 1:1 tachyarrhythmiadiscrimination.

FIG. 19 is a graph illustrating exemplary template and arrhythmicwaveforms used in the method of FIG. 18.

FIG. 20 is a flow chart illustrating a specific embodiment of the methodfor morphology-based 1:1 tachyarrhythmia discrimination as illustratedin FIG. 3 including an exemplary application of the method of FIG. 18.

FIG. 21 is a block diagram illustrating an embodiment of another 1:1tachyarrhythmia classifier being part of the morphology-based 1:1tachyarrhythmia discrimination circuit.

FIG. 22 is a block diagram illustrating a specific embodiment of the 1:1tachyarrhythmia classifier of FIG. 21.

FIG. 23 is a flow chart illustrating an embodiment of another method forclassifying 1:1 tachyarrhythmias for the morphology-based 1:1tachyarrhythmia discrimination.

FIG. 24 is a graph illustrating exemplary distribution of correlationcoefficients for an analysis using the method of FIG. 23.

FIG. 25 is a flow chart illustrating a specific embodiment of the methodfor morphology-based 1:1 tachyarrhythmia discrimination as illustratedin FIG. 3 including an exemplary application of the method of FIG. 23.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments in which the invention may bepracticed. These embodiments are described in sufficient detail toenable those skilled in the art to practice the invention, and it is tobe understood that the embodiments may be combined, or that otherembodiments may be utilized and that structural, logical and electricalchanges may be made without departing from the scope of the presentinvention. The following detailed description provides examples, and thescope of the present invention is defined by the appended claims andtheir equivalents.

In this document, the terms “a” or “an” are used, as is common in patentdocuments, to include one or more than one. In this document, the term“or” is used to refer to a nonexclusive or, unless otherwise indicated.Furthermore, all publications, patents, and patent documents referred toin this document are incorporated by reference herein in their entirety,as though individually incorporated by reference. In the event ofinconsistent usages between this documents and those documents soincorporated by reference, the usage in the incorporated reference(s)should be considered supplementary to that of this document; forirreconcilable inconsistencies, the usage in this document controls.

It should be noted that references to “an”, “one”, or “various”embodiments in this document are not necessarily to the same embodiment,and such references contemplate more than one embodiment.

A “circuit” in this document includes, but is not limited to, anapplication-specific circuit constructed to perform one or moreparticular functions or a general-purpose circuit programmed to performsuch function(s). Such a general-purpose circuit includes, but is notlimited to, a microprocessor or a portion thereof, a microcontroller orportions thereof, and a programmable logic circuit or a portion thereof.For example, a “comparator circuit” includes, among other things, anelectronic circuit comparator constructed to perform the only functionof a comparison between two signals or a portion of a general-purposecircuit driven by a code instructing that portion of the general-purposecircuit to perform the comparison between the two signals.

In this document, an “arrhythmic heart beat” includes a heart beatsensed during a detected tachyarrhythmia episode. An “arrhythmicwaveform” includes a waveform (such as a segment of an electrogram)associated with an arrhythmic heart beat. “Arrhythmic morphologicalfeatures” include morphological features of an arrhythmic waveform. An“arrhythmic feature vector” includes a vector associated with aplurality of arrhythmic morphological features of an arrhythmicwaveform. For example, an arrhythmic feature vector x=[x₁, x₂, . . .x_(N)], where x₁, x₂, . . . x_(N) are each associated with onearrhythmic morphology feature of the arrhythmic waveform, in a specificexample, x₁, x₂, . . . x_(N) are each an amplitude of the arrhythmicwaveform measured at the locations of the arrhythmic morphologicalfeatures. A “template heart beat” represents a heart beat associatedwith a known rhythm and used as a “template” for a morphologicalanalysis using morphological features associated with the known rhythm.The template heart beat may he produced from a plurality of hearts beatssensed during the known rhythm, such as by averaging. A “templatewaveform” includes a waveform associated with the template heart heat.“Template morphological features” include morphological features of thetemplate waveform. A “template feature vector” includes a vectorassociated with a plurality of template morphological features of thetemplate waveform. For example, a template feature vector y=[y₁, y₂, . .. y_(N)], where y₁, y₂, . . . y_(N) are each associated with onetemplate morphology feature of the template waveform. In a specificexample, y₁, y₂, . . . y_(N) are each an amplitude of the templatewaveform measured at the locations of the template morphologicalfeatures.

In this document, “mean” (such as in a “mean feature correlationcoefficient”) includes mean and other notations of central tendency,such as average and median.

This document discusses, among other things, a CRM system including acircuit for further classifying a detected cardiac arrhythmia including,but not being limited to, a 1:1 tachyarrhythmia. The 1:1tachyarrhythmia, characterized by an approximately one-to-oneassociation between atrial and ventricular depolarizations, is indicatedby substantially equal atrial and ventricular rates. The 1:1tachyarrhythmia is further classified based on a morphological analysisof arrhythmic waveforms and a template waveform each being a segment ofa cardiac signal such as an electrogram. It is to be understood thatwhile the classification of the 1:1 tachyarrhythmia is specificallydiscussed throughout this document as examples, the methods and.apparatuses according to the present subject matter are also applicablemorphology-based classification of cardiac arrhythmias other than the1:1 tachyarrhythmia.

FIG. 1 is an illustration of one embodiment of a CRM system 100 andportions of the environment in which CRM system 100 operates. CRM system100 includes an ICD 101 that is electrically coupled to a heart 199through leads 105 and 110. An external system 102 communicates with ICD101 via a telemetry link 103.

ICD 101 includes a hermetically sealed can housing an electronic circuitthat senses physiological signals and delivers therapeutic electricalpulses. The hermetically sealed can also functions as an electrode forsensing and/or pulse delivery purposes. In one embodiment, asillustrated in FIG. 1, the electronic circuit senses at least an atrialelectrogram and a ventricular electrogram from heart 199 and deliverspacing and cardioversion/defibrillation pulses to heart 199. Lead 105 isa pacing lead that includes a proximal end 106 connected to ICD 101 anda distal end 107 disposed in the right atrium (RA) of heart 199. Apacing-sensing electrode 108 is located at distal end 107. Anotherpacing-sensing electrode 109 is located near distal end 107. Electrodes108 and 109 are electronically connected to ICD 101 via separateconductors in lead 105 to allow sensing of the atrial electrogram and/ordelivery of atrial pacing pulses. Lead 110 is a defibrillation lead thatincludes a proximal end 111 connected to ICD 101 and a distal end 112disposed in the right ventricle (RV) of heart 199. A pacing-sensingelectrode 113 is located at distal end 112. A defibrillation electrode114 is located near distal end 112 but electrically separated frompacing-sensing electrode 113. Another defibrillation electrode 115 islocated at a distance from distal end 112 for supraventricularplacement. Electrodes 113, 114, and 115 are electrically connected toICD 101 via separate conductors in lead 110. Electrode 113 allowssensing of the ventricular electrogram and/or delivery of ventricularpacing pulses. Electrodes 114 and 115 allow delivery of ventricularcardioversion/defibrillation pulses.

ICD 101 includes a tachyarrhythmia detection and classification system120 that includes a morphology-based 1:1 tachyarrhythmia discriminationcircuit. An exemplary embodiment of a circuit of system 120 is discussedbelow with reference to FIG. 2. System 120 detects and classifies 1:1tachyarrhythmias by using a morphology-based 1:1 tachyarrhythmiadiscrimination method introduced below with reference to FIG. 3.Depending on the outcome of the tachyarrhythmia detection andclassification, system 120 determines whether to deliver a pacing and/orcardioversion/defibrillation therapy. In one embodiment, system 120delivers a ventricular defibrillation pulse when a 1:1 tachyarrhythmiais classified as a VT.

External system 102 allows for programming of ICD 101 and receivessignals acquired by ICD 101. In one embodiment, external system 102includes a programmer. In another embodiment, external system 102 is apatient management system including an external device in proximity ofICD 101, a remote device in a relatively distant location, and atelecommunication network linking the external device and the remotedevice. The patient management system allows access to ICD 101 from aremote location, such as for monitoring patient status and adjustingtherapies. In one embodiment, telemetry link 103 is an inductivetelemetry link. In an alternative embodiment, telemetry link 103 is afar-field radio-frequency telemetry link. Telemetry link 103 providesfor data transmission from ICD 101 to external system 102. This mayinclude, for example, transmitting real-time physiological data acquiredby ICD 101, extracting physiological data acquired by and stored in ICD101, extracting therapy history data stored in ICD 101, and extractingdata indicating an operational status of ICD 101 (e.g., battery statusand lead impedance). Telemetry link 103 also provides for datatransmission from external system 102 to ICD 101. This may include, forexample, programming ICD 101 to acquire physiological data, programmingICD 101 to perform at least one self-diagnostic test (such as for adevice operational status), programming ICD 101 to run a signal analysisalgorithm (such as an algorithm implementing the morphology-based 1:1tachyarrhythmia discrimination method discussed in this document), andprogramming ICD 101 to deliver pacing and/orcardioversion/defibrillation therapies.

FIG. 2 is a block diagram illustrating an embodiment of a circuit ofsystem 120. System 120 includes a sensing circuit 221, a rate detectioncircuit 222, a tachyarrhythmia detection circuit 223, a rhythmclassification circuit 224, and a morphology-based 1:1 tachyarrhythmiadiscrimination circuit 225. Sensing circuit 221 is electrically coupledto a heart to sense an atrial electrogram and a ventricular electrogramfrom the heart. The atrial electrogram includes atrial events, alsoknown as P waves, each indicative of an atrial depolarization. Theventricular electrogram includes ventricular events, also known as Rwaves, each indicative of a ventricular depolarization. Rate detectioncircuit 222 detects an atrial rate based on the atrial electrogram and aventricular rate based on the ventricular electrogram. The atrial rateis the frequency of the atrial events. The ventricular rate is thefrequency of the ventricular events. In one embodiment, the atrial andventricular rates are each expressed in beats per minute (bpm), i.e.,number of detected atrial or ventricular depolarizations per minute.Tachyarrhythmia detection circuit 223 detects a tachyarrhythmia based onat least one of the atrial rate and the ventricular rate. In oneembodiment, the tachyarrhythmia is detected when the atrial rate exceedsa predetermined tachyarrhythmia threshold rate. In another embodiment,the tachyarrhythmia is detected when the ventricular rate exceeds apredetermined tachyarrhythmia threshold rate. Rhythm classificationcircuit 224 classifies the detected tachyarrhythmia as a 1:1tachyarrhythmia when the atrial rate and the ventricular rate aresubstantially equal. In one embodiment, rhythm classification circuit224 classifies the detected tachyarrhythmia as the 1:1 tachyarrhythmiawhen the difference between the atrial rate and the ventricular rate isbetween a predetermined limit, such as 10 bpm. Morphology-based 1:1tachyarrhythmia discrimination circuit 225 further classifies the 1:1tachyarrhythmia, such as by its origin, by performing one or moremethods for morphology-based tachyarrhythmia discrimination discussed inthis document.

FIG. 3 is a flow chart illustrating an embodiment of a method 300 formorphology-based tachyarrhythmia discrimination. In one embodiment,method 300 is performed by morphology-based 1:1 tachyarrhythmiadiscrimination circuit 225. After a detected arrhythmia is classified asa 1:1 tachyarrhythmia at 310, a process of discriminating the 1:1tachyarrhythmia is started at 320.

Arrhythmia morphological features and template morphological featuresare collected at 330. The morphological features are points in a cardiacsignal that have morphological characteristics allowing discriminationbetween two or more types of 1:1 tachyarrhythmias. In one embodiment,the template heart beat represents a heart beat of a normal sinus rhythm(NSR). In one embodiment, template morphological features are collectedfrom the template heart beat and stored. This includes recording timingand other quantitative information, such as amplitudes, associated withthe features. In one specific embodiment, the feature collection isrepeated for a plurality of template heart beats, and the timing andother quantitative information associated with the features are averagescalculated over the plurality of template heart beats. Fordiscriminating the detected 1:1 tachyarrhythmia, arrhythmicmorphological features are extracted from the arrhythmic heart beat bytemporal correspondence with the template morphological features of thetemplate heart beat. A set of template morphological features and a setof corresponding arrhythmic morphological features are thus collectedfor the correlation analysis that follows. In another embodiment, atemplate waveform is stored. For discriminating the detected 1:1tachyarrhythmia, arrhythmic morphological features are collected fromthe arrhythmic heart beat. Then, template morphological features areextracted from the stored template waveform at the locations temporallycorresponding to the locations of the arrhythmic morphological featureson the arrhythmic waveform. A set of template morphological features anda set of arrhythmic morphological features are thus collected for thecorrelation analysis that follows. In one specific embodiment, thetemplate waveform is averaged over a plurality of template heart beats.

Correlation between the arrhythmic morphological features and thetemplate morphological features is analyzed at 340. The correlationanalysis results in one or more correlation coefficients associated witheach arrhythmic heart beat. One example for calculating such acorrelation coefficient, referred to as a feature correlationcoefficient (Fcc), is discussed in U.S. Pat. No. 6,708,058, “NORMALCARDIAC RHYTHM TEMPLATE GENERATION SYSTEM AND METHOD,” assigned toCardiac Pacemakers, Inc., which is hereby incorporated in its entirety.

The arrhythmic heart beat is classified based on the one or morecorrelation coefficients at 350. In one embodiment, each correlationcoefficient is compared to one or more thresholds defining detectionwindows each corresponding to one type of 1:1 tachyarrhythmia. Inanother embodiment, a score is produced based on the one or morecorrelation coefficients to provide a measure of the probability thatthe 1:1 tachyarrhythmia is of a known particular type. Examples of theknown particular types of 1:1 tachyarrhythmia include, but are notlimited to, supraventricular tachyarrhythmia (SVT), ventriculartachyarrhythmia (VT), monomorphic VT (MVT), and polymorphic VT (PVT).

The feature collection and correlation are repeated for a predeterminednumber of arrhythmic heart beats. If the predetermined number has notbeen reached at 350, steps 330 through 340 are repeated for the nextarrhythmic heart beat.

After the predetermined number has been reached at 360, the 1:1tachyarrhythmia is classified based on the classification given to theanalyzed arrhythmic heart beats at 370. In one embodiment, the 1:1tachyarrhythmia is classified by a majority voting. That is, the 1:1tachyarrhythmia is classified as a tachyarrhythmia of a particular typeif a majority of the analyzed arrhythmic heart beats are classified asthe tachyarrhythmia of that particular type. In one specific embodiment,80% (such as 8 out of 10 analyzed anhythmic heart beats) is consideredas the majority. For example, to discriminate between VT and SVT usingan NSR beat as the template heart beat, if 8 out of 10 arrhythmic heartbeats are classified as VT beats, the tachyarrhythmia is classified as aVT rhythm. Otherwise, it is classified as a SVT rhythm. In anotherspecific embodiment, 60% is considered as the majority. In anotherembodiment, in which a score is produced to provide a measure of thelikeliness that the 1:1 tachyarrhythmia is of a known particular type,the scores produced for all the analyzed arrhythmic heart beats areaveraged or otherwise processed to provide an indication for the type ofthe 1:1 tachyarrhythmia.

The discrimination of the 1:1 tachyarrhythmia is completed at 380, witha classification of the 1:1 tachyarrhythmia being indicated. In oneembodiment, the classification provides for a basis for making atherapeutic decision. For example, if a 1:1 tachyarrhythmia isclassified as a VT, a ventricular defibrillation pulse is delivered.

In one embodiment of step 330, a dynamic beat-driven morphologicalfeature extraction method is provided. Arrhythmic morphological featuresare collected from an arrhythmic waveform sensed while a 1:1tachyarrhythmia is indicated. The arrhythmic morphological featuresinclude points on the arrhythmic waveform that have detectablemorphological characteristics, such as points being or related to peakpoints and/or turning points. The temporal relationship between eachfeature and a fiducial point such as a peak of a depolarization is thendetermined for the arrhythmic heart beat. This temporal relationship isthen used to sample a pre-stored template waveform. This results in aset of arrhythmic morphological features and a corresponding set oftemplate morphological features. The dynamic beat-driven morphologicalfeature extraction is discussed in detail below, with reference to FIGS.5-9.

In one embodiment of steps 330 through 370, a template band-basedcorrelation analysis and a fuzzy discrimination process are provided.The template band is created for the correlation analysis based on themorphological variations in a plurality of template heart beats. Thetemplate band includes template morphological features with confidenceintervals. The correlation analysis is performed using the template bandand the arrhythmic morphological features extracted from an arrhythmicheart beat. This correlation analysis results in a plurality ofcorrelation coefficients representing a range of correlationcoefficients associated with that arrhythmic heart beat. A fuzzy scoreis then calculated based on the correlation coefficients for thearrhythmic heart beat. The 1:1 tachyarrhythmia is classified based onthe fuzzy scores calculated for all the analyzed arrhythmic heart beats.The template band-based correlation analysis and the fuzzydiscrimination are further discussed below, with reference to FIGS.10-15.

In another embodiment of step 340, a Mahalanobis distance-basedcorrelation analysis method is provided. The correlation analysisproduces Mahalanobis distance-based correlation coefficients for use inthe classification of the 1:1 tachyarrhythmia. The Mahalanobisdistance-based correlation analysis takes account the variability of andco-variability between template morphological features. It allows for amore robust classification of the 1:1 tachyarrhythmia than a Euclideandistance-based correlation analysis. The Mahalanobis distance-basedcorrelation analysis is further discussed below, with reference to FIGS.16-20.

In one embodiment of step 370, a method for discriminating the 1:1tachyarrhythmia based on stability of morphology is provided. Inaddition to the correlation analysis, the stability of the morphology isanalyzed to discriminate the 1:1 tachyarrhythmia. The variance of thecorrelation coefficients produced by the correlation analysis for theanalyzed arrhythmic heart beats is analyzed to discriminate the 1:1tachyarrhythmia. The morphological stability analysis is furtherdiscussed below, with reference to FIGS. 21-25.

It is to be understood that the embodiments discussed above are notnecessarily combined for use in a single CRM system. As those skilled inthe art will understand upon reading and comprehending this document, aCRM system for classifying 1:1 tachyarrhythmias may perform any one ormore of the dynamic beat-driven morphological feature extraction, thetemplate band-based correlation analysis and the fuzzy discrimination,the Mahalanobis distance-based correlation analysis, and themorphological stability analysis in the tachyarrhythmia detection andclassification.

FIG. 4 is a block diagram illustrating an embodiment of morphology-based1:1 tachyarrhythmia discrimination circuit 225. Morphology-based 1:1tachyarrhythmia discrimination circuit 225 includes a feature extractor430, a correlation analyzer 431, a 1:1 tachyarrhythmia classifier 432,and a beat counter 433. Feature extractor 430 extracts features awaveform associated with a heart beat. Correlation analyzer 431 computesa correlation coefficient between arrhythmic morphological features ofan arrhythmic beat of a 1:1 tachyarrhythmia and template morphologicalfeatures of a beat of a known type cardiac rhythm. In one embodiment,correlation analyzer 431 computes the feature correlation coefficient(Fcc) for each arrhythmic beat of a plurality of arrhythmic beats sensedduring a detected tachyarrhythmia. Beat counter 433 counts the number ofarrhythmic heart beats for which the arrhythmic features are extractedand analyzed. Based on the correlation coefficients calculated for apredetermined number of arrhythmic heart beats, 1:1 tachyarrhythmiaclassifier 432 classifies the 1:1 tachyarrhythmia.

In one embodiment, morphology-based 1:1 tachyarrhythmia discriminationcircuit 225 performs the method illustrated in FIG. 3. Feature extractor430 performs step 330, correlation analyzer 431 performs step 340, and1:1 tachyarrhythmia classifier 432 performs step 360.

Dynamic Beat-Driven Morphological Feature Extraction

FIG. 5 is a block diagram illustrating an embodiment of a featureextractor 530. Feature extractor 530 is one embodiment of featureextractor 430 and includes a template generation circuit 536, a templatewaveform storage circuit 537, a feature locating circuit 538, and afeature extracting circuit 539.

Feature extractor 530 selects arrhythmic morphological features on awaveform of art arrhythmic beat sensed during a detected 1:1tachyarrhythmia and extracts the temporally corresponding templatemorphological features from a stored template waveform. Templategeneration circuit 536 produces a template waveform to represent atemplate heart beat of a known type cardiac rhythm. Template waveformstorage circuit 537 stores that template waveform. After atachyarrhythmia is detected and classified as a 1:1 tachyarrhythmia,feature locating circuit 538 selects a plurality of arrhythmicmorphological features on an arrhythmic waveform of an arrhythmic heartbeat sensed during the 1:1 tachyarrhythmia. Feature locating circuit 538then produces timing information to indicate the locations of thesearrhythmic morphological features on the arrhythmic waveform. Featureextracting circuit 539 locates a plurality of template morphologicalfeatures on the stored template waveform based on the timing informationproduced by feature locating circuit 538.

FIG. 6 is a block diagram illustrating a feature extractor 630, which isa specific embodiment of feature extractor 530. Feature extractor 630includes a template generation circuit 636, template waveform storagecircuit 537, a feature locating circuit 638, and a feature extractingcircuit 639.

Template generation circuit 636 is a specific embodiment of templategeneration circuit 536 and includes a template waveform alignmentcircuit 640 and a template averaging circuit 641. Template waveformalignment circuit 640 aligns a plurality of template waveforms by afiducial point. The template waveforms are each associated with a heartbeat sensed during a known type cardiac rhythm, such as an NSR. In oneembodiment, template waveform alignment circuit 640 includes a peakdetector to detect a peak of a depolarization for use as the fiducialpoint. Template averaging circuit 641 produces the template waveformthat will be stored in template waveform storage circuit 537 byaveraging the amplitudes of the plurality of template waveforms.

Feature locating circuit 638 is a specific embodiment of featurelocating circuit 538 and includes a waveform input 642, a fiducial pointlocating circuit 643, a feature selecting circuit 644, and a featuretiming circuit 645. Waveform input 642 receives the arrhythmic waveform.Fiducial point locating circuit 643 detects a fiducial point on thearrhythmic waveform. In one embodiment, fiducial point locating circuit643 includes a peak detector to detect a peak of a depolarization fromthe arrhythmic waveform. In one embodiment, during the generation of thetemplate waveform that will be stored in template waveform storagecircuit 537, waveform input 642 also receives the template waveforms,and fiducial point locating circuit 643 also detects the fiducial pointon each of the template waveforms for the alignment of the templatewaveforms. Feature selecting circuit 644 selects a plurality ofarrhythmic morphological features on the arrhythmic waveform based onone or more predetermined criteria. Example of the morphologicalfeatures include, but are not limited to, peaks and other turning pointson the arrhythmic waveform, points associated with maximum slopes, andpoints having a predefined timing relationship with the peaks, otherturning points, and/or points associated with maximum slopes. Featuretiming circuit 645 measures time intervals each between the fiducialpoint on the arrhythmic waveform and one of the selected arrhythmicmorphological features.

Feature extracting circuit 639 is a specific embodiment of featureextracting circuit 539 and locates the plurality of templatemorphological features on the template waveform based on the fiducialpoint on the arrhythmic waveform and the measured time intervals.Feature extracting circuit 639 locates a corresponding fiducial point onthe template waveform, aligns the fiducial point on the arrhythmicwaveform and the corresponding fiducial point on the template waveform,and locates the template morphological features on the template waveformusing the measured time intervals. Each measured time interval is usedas a time interval between a template morphological feature and thefiducial point on the template waveform.

FIG. 7 is a flow chart illustrating an embodiment of a method 700 forextracting morphological features for the morphology-based 1:1tachyarrhythmia discrimination. In one embodiment, feature extractor530, including its specific embodiment, feature extractor 630, performsmethod 700.

A template waveform is produced at 710. The template waveform representsa template heart beat. In one embodiment, multiple waveforms are alignedby a fiducial point and averaged to produce the template waveform. Eachwaveform is associated with a heart beat that occurs during a known typecardiac rhythm. In one specific embodiment, about 16 waveforms arealigned by a peak of depolarization and averaged to produce the templatewaveform. Each of the 16 waveforms represents a heart beat that occursduring an NSR. The template waveform is stored at 720 for use in themorphology-based 1:1 tachyarrhythmia discrimination when atachyarrhythmia is detected and classified as a 1:1 tachyarrhythmia.

An arrhythmic waveform is received at 730. The arrhythmic waveformrepresents an arrhythmic heart beat that occurs during a 1:1tachyarrhythmia. A plurality of arrhythmic morphological features areselected from the arrhythmic waveform based on predetermined criteria at740. The arrhythmic morphological features are characteristic pointsthat are reliably detectable form substantially all the arrhythmicwaveforms used in the morphology-based 1:1 tachyarrhythmiadiscrimination. Timing information indicative of the locations of thearrhythmic morphological features is produced at 750. In one embodiment,a fiducial point, such as a peak of a depolarization, is detected fromthe arrhythmic waveform. The timing information includes time intervalseach measured between the fiducial point and one of the arrhythmicmorphological features.

A plurality of template morphological features are located on thetemplate waveform based on the timing information indicative of thelocations of the plurality of arrhythmic morphological features at 760.In one embodiment, the template morphological features are located onthe template waveform based on the fiducial point and the measured timeintervals. A fiducial point corresponding to the fiducial point on thearrhythmic waveform is located on the template waveform. The fiducialpoints on the arrhythmic and template waveforms are aligned. Thetemplate morphological features are located on the template waveformusing the measured time intervals. Each measured time interval is usedas an interval between one template morphological feature and thefiducial point on the template waveform.

FIG. 8 is a graph illustrating feature extraction using method 700.Waveform 800 is an exemplary template waveform. Waveform 801 is anexemplary arrhythmic waveform. Waveforms 800 and 801 are each a segmentof a ventricular electrogram showing a ventricular depolarization(R-wave). The two waveforms are temporally aligned by the peak of theR-wave. Arrhythmic morphological features 811A-811H are selected onwaveform 801. Corresponding template morphological features 810A-810Hare then extracted from waveform 800 as points that are temporally(vertically as shown in FIG. 8) aligned with arrhythmic morphologicalfeatures 811A-811H.

FIG. 9 is a flow chart illustrating a method 900 for themorphology-based tachyarrhythmia discrimination. Method 900 is aspecific embodiment of method 300 and includes an exemplary applicationof method 700. Step 930 in method 900 is a specific embodiment of step330 and includes method 700 as discussed above.

Feature extraction using method 700 in a morphology-basedtachyarrhythmia discrimination method such as method 300 has a number ofadvantages. For example, features extracted from the arrhythmic waveformrepresent the morphological characteristics of the arrhythmic heart beatwith high fidelity. In general, an arrhythmic heart beat ismorphologically more complex and less organized than a template heartbeat that is a beat of the NSR. Therefore, the locations of thearrhythmic morphological features determined from the arrhythmicwaveform represent the morphology of the template heart beat welt,possibly with some harmless redundancy in morphological representation.Method 700 also allows variable number of morphological features to beused in the analysis of feature correlation at 340 after the templatewaveform is generated and stored.

Template Band-Based Correlation Analysis and Fuzzy Discrimination

FIG. 10 is a block diagram illustrating an embodiment of a correlationanalyzer 1031 and a 1:1 tachyarrhythmia classifier 1032. Correlationanalyzer 1031 is an embodiment of correlation analyzer 431, and 1:1tachyarrhythmia classifier 1032 is an embodiment of 1:1 tachyarrhythmiaclassifier 432. Correlation analyzer 1031 includes a feature vectorgeneration circuit 1048 and a correlation computing circuit 1049, and1:1 tachyarrhythmia classifier 1032 includes a beat classificationcircuit 1050.

Feature vector generation circuit 1048 produces a template featurevector (a), an arrhythmic feature vector (b), a maximum feature vector(a_(max)), and a minimum feature vector (a_(min)). The template featurevector (a) is associated with a plurality of template morphologicalfeatures of a plurality of template heart beats of a known type cardiacrhythm. The arrhythmic feature vector (b) is associated with a pluralityof arrhythmic morphological features of an arrhythmic heart beat of a1:1 tachyarrhythmia. The maximum feature vector (a_(max)) and theminimum feature vector (a_(min)) are each the sum of the templatefeature vector (a) and a deviation vector, and are produced based on thetemplate morphological features and the arrhythmic morphologicalfeatures. Each deviation vector is a measure of beat-to-beatmorphological variations in the plurality of template heart beats.Correlation computing circuit 1049 computes a mean feature correlationcoefficient (Fcc_(mean)) based on the template feature vector (a) andthe arrhythmic feature vector (b), a maximum feature correlationcoefficient (Fcc_(max)) based on the maximum feature vector (a_(max))and the arrhythmic feature vector (b), and a minimum feature correlationcoefficient (Fcc_(min)) based on the minimum feature vector (a_(min))and the arrhythmic feature vector (b). Beat classification circuit 1050classifies the arrhythmic heart beat based on the mean featurecorrelation coefficient (Fcc_(mean)), the maximum feature correlationcoefficient (Fcc_(max)), the minimum feature correlation coefficient(Fcc_(min)), and at least one predetermined correlation threshold.

FIG. 11 is a block diagram illustrating a correlation analyzer 1131,which is a specific embodiment of correlation analyzer 1031, and a 1:1tachyarrhythmia classifier 1132, which is a specific embodiment of 1:1tachyarrhythmia classifier 1032. Correlation analyzer 1131 includes afeature vector generation circuit 1148 and a correlation computingcircuit 1149, and 1:1 tachyarrhythmia classifier 1132 includes a beatclassification circuit 1150 and an episode classification circuit 1165.

Feature vector generation circuit 1148 is a specific embodiment offeature vector generation circuit 1048 and includes a template bandconstruction circuit 1152, an arrhythmic feature vector generationcircuit 1153, a deviation vector generation circuit 1154, and a boundaryfeature vector generation circuit 1155. Template band constructioncircuit 1152 produces the template feature vector (a) and a templatestandard deviation vector (σ) based on the morphological features of theplurality of template heart beats. Template band construction circuit1152 includes a template vector generator 1157, a mean vector calculator1158, and a standard deviation vector calculator 1159. Template vectorgenerator 1157 produces a plurality of template feature vectors eachrepresentative of a plurality of template morphological features of onetemplate heart beat of the plurality of template heart beats. Meanvector calculator 1158 produces the template feature vector (a), whichis a mean vector of the plurality of template feature vectors. Standarddeviation vector calculator 1159 produces the template standarddeviation vector (σ), which is a standard deviation vector of theplurality of template feature vectors. Arrhythmic feature vectorgeneration circuit 1153 produces the arrhythmic feature vector (b) basedon the morphological features of the arrhythmic heart beat of the 1:1tachyarrhythmia. Deviation vector generation circuit 1154 produces amaximum deviation vector (x_(max)) and a minimum deviation vector(x_(min)) based on the template feature vector (a), the arrhythmicfeature vector (b), and the template standard deviation vector (σ). Inone embodiment, the maximum deviation vector (x_(max)) and a minimumdeviation vector (x_(min)) are calculated based on:

$\begin{matrix}{{{\underset{\_}{x}}_{\max} = {{\frac{\underset{\_}{b} - \underset{\_}{a}}{{{\underset{\_}{b} - \underset{\_}{a}}}_{\infty}} \cdot}*\underset{\_}{\delta}}},{and}} & \lbrack 1\rbrack \\{{{\underset{\_}{x}}_{\min} = {{\frac{\underset{\_}{b} - \underset{\_}{a}}{{{\underset{\_}{b} - \underset{\_}{a}}}_{\infty}} \cdot}*\underset{\_}{\delta}}},} & \lbrack 2\rbrack\end{matrix}$

where δ is a feature variation vector (δ=kσ, where k is a scalarconstant dependent on the desired confidence level), ∥ ∥_(∞) the maximumof the absolute value, and “·*” is the operator for anelement-by-element product. Boundary feature vector generation circuit1155 produces the maximum feature vector (a_(max)) by adding the maximumdeviation vector (x_(max)) to the template feature vector (a) andproduces the minimum feature vector (a_(min)) by adding the minimumdeviation vector (x_(min)) to the template feature vector (a). That is:

a _(max) =a+x _(max), and   [3]

a _(min) =a+x _(min),   [4]

Correlation computing circuit 1149 is a specific embodiment ofcorrelation computing circuit 1049 and computes feature correlationcoefficients including a mean feature correlation coefficient(Fcc_(mean)), a maximum feature correlation coefficient (Fcc_(max)), anda minimum feature correlation coefficient (Fcc_(min)). In oneembodiment, as discussed in U.S. Pat. No. 6,708,058, a featurecorrelation coefficient is computed using the following equation:

$\begin{matrix}{{{Fcc} = {{{FCC}\left( {\underset{\_}{a},\underset{\_}{b}} \right)} = \frac{\left( {{N\; {\sum\limits_{i = 1}^{N}{{\underset{\_}{a}}_{i}{\underset{\_}{b}}_{i}}}} - {\left( {\sum\limits_{i = 1}^{N}{\underset{\_}{a}}_{i}} \right)\left( {\sum\limits_{i = 1}^{N}{\underset{\_}{b}}_{i}} \right)}} \right)^{2}}{\left( {{N\; {\sum\limits_{i = 1}^{N}{\underset{\_}{a}}_{i}^{2}}} - \left( {\sum\limits_{i = 1}^{N}{\underset{\_}{a}}_{i}} \right)^{2}} \right)\left( {{N{\sum\limits_{i = 1}^{N}{\underset{\_}{b}}_{i}^{2}}} - \left( {\sum\limits_{i = 1}^{N}b_{i}} \right)^{2}} \right)}}},} & \lbrack 5\rbrack\end{matrix}$

where IN is the number of morphological features extracted from eachtemplate or arrhythmic heart beat, a_(i) is associated with the i^(th)template morphological feature, and b_(i) is associated with the i^(th)arrhythmic morphological feature. In one specific example, N=8.Correlation computing circuit 1149 computes feature correlationcoefficients using the following equations:

Fcc _(mean) =FCC(a,b),   [6]

Fcc _(max) =FCC(a _(max) ,b),   [7]

Fcc _(min) =FCC(a _(min) b,),   [8]

Beat classification circuit 1150 is a specific embodiment of beatclassification circuit 1050 and include a fizzy decision circuit 1164.Fuzzy decision circuit 1164 calculate a fuzzy score for the arrhythmicheart beat based on the mean feature correlation coefficient(Fcc_(mean)), the maximum feature correlation coefficient (Fcc_(max)),the minimum feature correlation coefficient (Fcc_(min)), and the atleast one predetermined correlation threshold (Fcc_(th)). The fuzzyscore represents an estimated probability of the 1:1 tachyarrhythmiabeing a known type tachyarrhythmia. Additional details of the fuzzyscore calculation is discussed below with reference to FIG. 14.

Episode classification circuit 1165 classifies the 1:1 tachyarrhythmiabased on the fuzzy scores produced for a plurality of arrhythmic heartbeats of the 1:1 tachyarrhythmia. The plurality of arrhythmic heartbeats represent an “episode” of the 1:1 tachyarrhythmia. Episodeclassification circuit 1165 includes a fuzzy score averaging circuit1166 and a comparator 1167. Fuzzy score averaging circuit 1166calculates an episode fuzzy score being an average of the fuzzy scoresproduced by correlation computing circuit 1149 for the plurality ofarrhythmic heart beats. Comparator 1167 compares the episode fuzzy scoreto a predetermined classification threshold and classifies the 1:1tachyarrhythmia based on an outcome of the comparison. In oneembodiment, comparator 1167 classifies the 1:1 tachyarrhythmia as aknown first type tachyarrhythmia if the episode fuzzy score exceeds thepredetermined classification threshold and a known second typetachyarrhythmia if the episode fuzzy score does not exceed thepredetermined classification threshold.

FIG. 12 is a flow chart illustrating an embodiment of a method 1200 foranalyzing correlation and classifying 1:1 tachyarrhythmias for themorphology-based 1:1 tachyarrhythmia discrimination. In one embodiment,correlation analyzer 1031 and 1:1 tachyarrhythmia classifier 1032,including their specific embodiment, correlation analyzer 1131 and 1:1tachyarrhythmia classifier 1132, perform method 1200.

A template feature vector (a) and a template standard deviation vector(a) are produced based on morphological features of a plurality oftemplate heart beats of a known type cardiac rhythm at 1210. In oneembodiment, a plurality of template feature vectors each representativeof a plurality of template morphological features of one of the templateheart beats are received. The template feature vector (a) is a meanvector of the plurality of template feature vectors. The templatestandard deviation vector (σ) is a standard deviation vector of theplurality of template feature vectors.

An arrhythmic feature vector (b) is produced based on morphologicalfeatures of an arrhythmic heart beat of a 1:1 tachyarrhythmia at 1220.In one embodiment, an arrhythmic waveform representing a heart beat ofthe 1:1 tachyarrhythmia is received. A plurality of morphologicalfeatures are extracted from the arrhythmic waveform. The arrhythmicfeature vector (b) is associated with the plurality of morphologicalfeatures.

A maximum deviation vector (x_(max)) and a minimum deviation vector(x_(min)) are produced based on the template feature vector (a), thearrhythmic feature vector (b), and the template standard deviationvector (σ), using equations [1] and [2], at 1230. A maximum featurevector a_(max)) and a minimum feature vector (a_(min)) are produced at1240. The maximum feature vector (a_(max)) is calculated by adding themaximum deviation vector (x_(max)) to the template feature vector (a),i.e., using equation [3]. The minimum feature vector (a_(min)) iscalculated by adding the minimum deviation vector (x_(min)) to thetemplate feature vector (a), using equation [4].

A mean feature correlation coefficient (Fcc_(mean)), a maximum featurecorrelation coefficient (Fcc_(max)), and a minimum feature correlationcoefficient (Fcc_(min)) are computed at 1250. The maximum featurecorrelation coefficient (Fcc_(max)) is computed based on the maximumfeature vector (a_(max)) and the arrhythmic feature vector (b) usingequation [6]. The mean feature correlation coefficient (Fcc_(mean)) iscomputed based on the template feature vector (a) and the arrhythmicfeature vector (b) using equation [7]. The minimum feature correlationcoefficient (Fcc_(min)) is computed based on the minimum feature vectorand the arrhythmic feature vector (b) using equation [8].

The arrhythmic heart beat is classified based on the mean featurecorrelation coefficient (Fcc_(mean)), the maximum feature correlationcoefficient (Fcc_(max)), the minimum feature correlation coefficient(Fcc_(min)), and at least one predetermined correlation threshold(Fcc_(th)) at 1260. In one embodiment, a fuzzy score for the arrhythmicheart beat is calculated based on based on the mean feature correlationcoefficient (Fcc_(mean)), the maximum feature correlation coefficient(Fcc_(max)), the minimum feature correlation coefficient (Fcc_(min)),and the predetermined correlation threshold (Fcc_(th)). The fuzzy scorerepresents an estimated probability of the 1:1 tachyarrhythmia being aknown type tachyarrhythmia.

In a further embodiment, the 1:1 tachyarrhythmia is classified based onthe fuzzy scores produced for a plurality of arrhythmic heart beats ofthe 1:1 tachyarrhythmia. An episode fuzzy score is calculated byaveraging the fuzzy scores produced for the plurality of arrhythmicheart beats and is compared to a predetermined classification threshold.The 1:1 tachyarrhythmia is classified as a known first typetachyarrhythmia if the episode fuzzy score exceeds the predeterminedclassification threshold and as a known second type tachyarrhythmia ifthe episode fuzzy score does not exceed the predetermined classificationthreshold. In one specific example, the known type cardiac rhythm is anNSR, the known first type tachyarrhythmia is an SVT, and the knownsecond type tachyarrhythmia is a VT.

FIG. 13 is a graph illustrating an exemplary template band as used inmethod 1200. As illustrated, the template band includes a mean waveform1300 with two curves 1301 and 1302 representing the confidence levels(a+δ and a−δ). Each morphological feature is associated with a meanpoint being one of points 1310A-H and a range between a maximum pointbeing one of points 1311A-H and a minimum point being one of points1312A-H. The mean feature vector (a) is associated with points1310A-1310H on mean waveform 1300. The maximum feature vector (a_(max))and the minimum feature vector (a_(min)) are within the template band.

FIGS. 14A-D include graphs illustrating a fuzzy decisional process forclassifying the arrhythmia beat in step 1260 of method 1200. A curve1400 represents an estimate of the distribution of the probability thata detected 1:1 tachyarrhythmia is of a particular type over the featurecorrelation coefficient. As illustrated, the probability distributesbetween the maximum feature correlation coefficient (Fcc_(max)) and theminimum feature correlation coefficient (Fcc_(min)), and peaks at themean feature correlation coefficient (Fcc_(mean)). The fuzzy score isthe ratio of the shaded area to the area of the triangle between theestimated probability distribution curve and the Fcc axis. FIG. 14Aillustrates the scenario that the predetermined correlation threshold(Fcc_(th)) is greater than the maximum feature correlation coefficient(Fcc_(max)). The corresponding fuzzy score is 1. FIG. 14B illustratesthe scenario that the predetermined correlation threshold (Fcc_(th)) issmaller than the minimum feature correlation coefficient (Fcc_(min)).The corresponding fuzzy score is 0. FIG. 14C illustrates the scenariothat the predetermined correlation threshold (Fcc_(th)) is between themean feature correlation coefficient (Fcc_(mean)) and the maximumfeature correlation coefficient (Fcc_(max)). The corresponding fuzzyscore is given as:

$\begin{matrix}{{fuzzy\_ score} = {1 - {\frac{\left( {{Fcc}_{\max} - {Fcc}_{th}} \right)^{2}}{\left( {{Fcc}_{\max} - {Fcc}_{mean}} \right) \cdot \left( {{Fcc}_{\max} - {Fcc}_{\min}} \right)}.}}} & \lbrack 9\rbrack\end{matrix}$

FIG. 14D illustrates the scenario that the predetermined correlationthreshold (Fcc_(th)) is between the minimum feature correlationcoefficient (Fcc_(min)) and the mean feature correlation coefficient(Fcc_(mean)). The corresponding fuzzy score is given as:

$\begin{matrix}{{fuzzy\_ score} = {1 - {\frac{\left( {{Fcc}_{th} - {Fcc}_{\min}} \right)^{2}}{\left( {{Fcc}_{mean} - {Fcc}_{\min}} \right) \cdot \left( {{Fcc}_{\max} - {Fcc}_{\min}} \right)}.}}} & \lbrack 10\rbrack\end{matrix}$

If the fuzzy score is 1, the arrhythmic heart beat is classified as aknown first type tachyarrhythmia. If the fuzzy score is 0, thearrhythmic heart beat is classified as a known second typetachyarrhythmia. If the fuzzy score is between 0 and 1, the fuzzy scoreindicates the probability that the arrhythmic heart beat is the knownfirst type tachyarrhythmia, and the arrhythmic heart beat is classifiedby giving the probability. In one specific example, the known typecardiac rhythm is an NSR, the known first type tachyarrhythmia is a VT,and the known second type tachyarrhythmia is an SVT.

FIG. 15 is a flow chart illustrating a method 1500 for themorphology-based tachyarrhythmia discrimination. Method 1500 is anotherspecific embodiment of method 300 and includes an exemplary applicationof method 1200. Step 1540 in method 1500 is a specific embodiment ofstep 340 and includes steps 1210-1250 of method 1200 as discussed above.Step 1550 in method 1500 is a specific embodiment of step 350 andincludes step 1260 of method 1200 as discussed above. Step 1570 inmethod 1500 is a specific embodiment of step 370. At 1570, the 1:1tachyarrhythmia is classified based on fuzzy scores calculated for aplurality of arrhythmic heart beats sensed during the 1:1tachyarrhythmia. In one embodiment, the fuzzy scores are averaged andcompared to a predetermined classification threshold. The 1:1tachyarrhythmia is classified based on the outcome of the comparison.

Because of the variations in the morphology associated with a heart beatof a known rhythm such as the NSR, an averaged waveform does not alwaysprovide an unbiased template for the morphology-based tachyarrhythmiadiscrimination. Correlation analysis and tachyarrhythmia classificationusing method 1200 in a morphology-based tachyarrhythmia discriminationmethod such as method 300 takes into account the variation anduncertainty of a template waveform. The fuzzy score reflects suchvariation and uncertainty. The classification of a 1:1 tachyarrhythmiausing the episode fuzzy score can be conceptualized as beingadvantageously based on a value fusion rather than a decision fusion.

Mahalanobis Distance-Base Correlation Analysis

FIG. 16 is a block diagram illustrating an embodiment of a correlationanalyzer 1631. Correlation analyzer 1631 is another embodiment ofcorrelation analyzer 431 and includes a template circuit 1670, anarrhythmic feature vector generation circuit 1671, and a correlationcomputing circuit 1672. Correlation analyzer 1631 performs a Mahalanobisdistance-based correlation analysis between an arrhythmic waveform and atemplate waveform.

Template circuit 1670 produces a template feature vector and an inversecovariance matrix (W) of a template feature matrix (X) based on aplurality of template heart beats of a known type cardiac rhythm. Eachtemplate heart beat is represented by a template waveform including aplurality of template morphological features. The template featurevector (x), the template feature matrix (X), and the inverse covariancematrix (NV) are each produced based on the template morphologicalfeatures of the plurality of template heart beats. Arrhythmic featurevector generation circuit 1671 produces an arrhythmic feature vector (y)based on a plurality of arrhythmic morphological features associatedwith an arrhythmic heart beat of a 1:1 tachyarrhythmia. Correlationcomputing circuit 1672 produces a Mahalanobis distance-based featurecorrelation coefficient (mFcc) for the arrhythmic heart beat based onthe template feature vector (x), the arrhythmic feature vector (y), andthe inverse covariance matrix (W). The Mahalanobis distance-basedfeature correlation coefficient (mFcc) is computed based on equation[11]:

$\begin{matrix}{{{mFcc} = \frac{\left( {{\underset{\_}{y}}^{T}\underset{\_}{Wx}} \right)^{2}}{{\underset{\_}{y}}^{T}\underset{\_}{W}\; {\underset{\_}{y} \cdot {\underset{\_}{x}}^{T}}\underset{\_}{Wx}}},} & \lbrack 11\rbrack\end{matrix}$

where x^(T) is the transposed x, and y^(T) is the transposed y.

FIG. 17 is a block diagram illustrating an embodiment of a correlationanalyzer 1731, which is a specific embodiment of correlation analyzer1631. Correlation analyzer 1731 includes a template circuit 1770, anarrhythmic feature vector generation circuit 1771, and correlationcomputing circuit 1672.

Template circuit 1770 is a specific embodiment of template circuit 1670and includes a feature location vector generation circuit 1775, afeature vector generation circuit 1776, a mean feature vector generationcircuit 1777, a feature matrix generation circuit 1778, an inversecovariance matrix generation circuit 1780, and a template storagecircuit 1784. Feature location vector generation circuit 1775 produce afeature location vector (p) indicative of the locations of templatemorphological features associated with each of the plurality of templateheart beats. Feature vector generation circuit 1776 produces a featurevector for each of the plurality of template heart beats. The featurevector includes parameters measured from a plurality of templatemorphological features located for that template heart beat. Meanfeature vector generation circuit 1777 produces the template featurevector (x) as the mean vector of the feature vectors produced for theplurality of template heart beats. Feature matrix generation circuit1778 produces the template feature matrix (X) including all the featurevectors produced for the plurality of template heart beats. That is:

$\begin{matrix}{{\underset{\_}{X} = \begin{bmatrix}x_{11} & x_{12} & \ldots & x_{1\; N} \\x_{21} & x_{22} & \ldots & x_{2\; N} \\\vdots & \vdots & \ddots & \vdots \\x_{M\; 1} & x_{M\; 2} & \ldots & x_{MN}\end{bmatrix}},} & \lbrack 12\rbrack\end{matrix}$

where N is the number of template heart beats in the plurality oftemplate heart beats and M is the number of the template morphologicalfeatures associated with each template heart beat of the plurality oftemplate heart beats.

Inverse covariance matrix generation circuit 1780 computes the inversecovariance matrix (W) of the template feature matrix (X). Inversecovariance matrix generation circuit 1780 includes a covariance matrixgeneration circuit 1781, a matrix reduction circuit 1782, and an inversematrix generation circuit 1783. Covariance matrix generation circuit1781 computes a covariance matrix (Σ) of the template feature matrix (X)based on equation [13]:

$\begin{matrix}{{\underset{\_}{\Sigma} = \begin{bmatrix}\sigma_{1}^{2} & \sigma_{12}^{2} & \ldots & \sigma_{1\; M}^{2} \\\sigma_{21}^{2} & \sigma_{2}^{2} & \ldots & \sigma_{2\; M}^{2} \\\vdots & \vdots & \ddots & \vdots \\\sigma_{M\; 1}^{2} & \sigma_{M\; 2}^{2} & \ldots & \sigma_{M}^{2}\end{bmatrix}},} & \lbrack 13\rbrack\end{matrix}$

where σ_(i) ² is the variance of the i^(th) template morphologicalfeature, and σ_(ij) ² is the covariance of between the i^(th) templatemorphological feature and the j^(th) template morphological feature.Matrix reduction circuit 1782 produces a reduced covariance matrix (Σ′)by setting all non-diagonal elements of the covariance matrix (Σ) tozero. That is:

$\begin{matrix}{{\underset{\_}{\Sigma}}^{\prime} = {\begin{bmatrix}\sigma_{1}^{2} & 0 & \ldots & 0 \\0 & \sigma_{2}^{2} & \ldots & 0 \\\vdots & \vdots & \ddots & \vdots \\0 & 0 & \ldots & \sigma_{N}^{2}\end{bmatrix}.}} & \lbrack 14\rbrack\end{matrix}$

Inverse matrix generation circuit 1783 produces the inverse covariancematrix (W) being an inverse covariance matrix of the reduced covariancematrix (Σ′). That is:

$\begin{matrix}{{\underset{\_}{W} = \begin{bmatrix}\frac{1}{\sigma_{1}^{2} + \delta} & 0 & \ldots & 0 \\0 & \frac{1}{\sigma_{2}^{2} + \delta} & \ldots & 0 \\\vdots & \vdots & \ddots & \vdots \\0 & 0 & \ldots & \frac{1}{\sigma_{N}^{2} + \delta}\end{bmatrix}},} & \lbrack 15\rbrack\end{matrix}$

where δ is a small regularization factor added to prevent the inversecovariance matrix (W) from being singular. In one specific embodiment, δis set to one half of the smallest σ_(i) ². Template storage circuit1784 stores the template feature vector (x), the feature location vector(p), and the inverse covariance matrix (W) for the correlation analysiswhen a tachyarrhythmia is detected and classified as a 1:1tachyarrhythmia.

Arrhythmic feature vector generation circuit 1771 is a specificembodiment of arrhythmic feature vector generation circuit 1671 andincludes a feature input and a feature extraction circuit. The featureinput receives an arrhythmic waveform representative of the arrhythmicheart beat of the 1:1 tachyarrhythmia. The feature extraction circuitextracts a plurality of arrhythmic morphological features form thatarrhythmic waveform based on the feature location vector (p) andproduces the arrhythmic feature vector (y).

FIG. 18 is a flow chart illustrating an embodiment of a method 1800 foranalyzing correlation for the morphology-based 1:1 tachyarrhythmiadiscrimination. In one embodiment, method 1800 is performed bycorrelation analyzer 1631, including its specific embodiment,correlation analyzer 1731.

A feature location vector (p), a template feature vector (x), and aninverse covariance matrix (W) of a template feature matrix are receivedat 1810. These vectors and matrix are associated with a plurality oftemplate heart beats of a known type cardiac rhythm. In one embodiment,the known type cardiac rhythm includes an NSR. Each template heart beatis represented by a template waveform having a plurality of templatemorphological features. In one embodiment, a plurality of templatemorphological features are located for each template heart beat. Thefeature location vector (p) is produced to indicate the locations of thetemplate morphological features for all the template heart beats. Afeature vector is produced for each template heart beat based on thetemplate morphological features located for that template heart beat. Atemplate feature vector (x) is produced as the mean vector of all thefeature vectors produced for the plurality of template heart beats. Atemplate feature matrix (X) is produced to include all the featurevectors produced, as shown in equation [12]. A covariance matrix (Σ) ofthe template feature matrix (X) is computed using equation [13]. Areduced covariance matrix (Σ′) is produced by setting all non-diagonalelements of the covariance matrix (Σ) to zero, as shown in equation[14]. The inverse covariance matrix (W) is produced as an inversecovariance matrix of the reduced covariance matrix (Σ′), as shown inequation [15]. After being produced, at least the template featurevector (x), the feature location vector (p), and the inverse covariancematrix (W) are stored for use in the Mahalanobis distance-basedcorrelation analysis when a 1:1 tachyarrhythmia is detected.

An arrhythmic waveform representative of an arrhythmic heart beat of a1:1 tachyarrhythmia is received at 1820. A plurality of arrhythmicmorphological features are extracted from the arrhythmic waveform basedon at least the feature location vector (p) at 1830. An arrhythmicfeature vector (y) associated with the arrhythmic heart beat is producedat 1840. A Mahalanobis distance-based correlation coefficient (mFcc) forthe each arrhythmic heart beat is produced based on the template featurevector (x), the arrhythmic feature vector (y), and the inversecovariance matrix (W), using equation [11], at 1850.

FIG, 19 is a graph illustrating exemplary template and arrhythmicwaveforms showing an advantage of using method 1800. A template waveform1900 is the template waveform being an averaged waveform representingthe plurality of template heart beats of the known type cardiac rhythm.An arrhythmic waveform 1901 is the waveform of the arrhythmic heart beatsensed during the it 1:1 tachyarrhythmia. Template waveform 1900includes template morphological features 1910A-H. Arrhythmic waveform1901 includes arrhythmic morphological features 1911A-H, which aretemporally corresponding to template morphological features 1910A-H. Inthe Mahalanobis distance-based correlation analysis, a templatemorphology feature associated with a relatively large variability amongthe plurality of template heart beats has a relatively smaller weight inthe computation of the Mahalanobis distance-based correlationcoefficient (mFcc). In the example of FIG. 19, template morphologicalfeature 1910F has a relatively large variation among the plurality oftemplate heart beats. Thus, even though arrhythmic morphological feature1911F differs significantly from template morphological feature 1910F interms of amplitude, the Mahalanobis distance-based correlationcoefficient (mFcc) is 0.99. That is, the arrhythmic heart beat and thetemplate heart beat as shown in FIG. 19 are substantially correlated. Inone specific embodiment, as illustrated in FIG. 19, the template heartbeat is a heart beat of an NSR, and the arrhythmic heart beat of the 1:1tachyarrhythmia is discriminated between an SVT heart beat and a VTheart beat. The Mahalanobis distance-based correlation coefficient(mFcc) of 0.99 supports a classification of an SVT heart beat.

FIG. 20 is a flow chart illustrating a method 2000 for themorphology-based tachyarrhythmia discrimination. Method 2000 is aspecific embodiment of method 300 and includes an exemplary applicationof method 1800. Step 2040 in method 2000 is a specific embodiment ofstep 340 and includes method 1800 as discussed above.

The Mahalanobis distance-based correlation analysis takes into accountthe variability of and co-variability between morphological features.Instead of treating all template morphological features equally, theweight of each template morphological feature in the computation of theMahalanobis distance-based correlation coefficient (mFcc) depends on thevariability of that template morphological feature as measured from theplurality of template heart beats. The purpose is to prevent normalvariations in the morphology of a heart beat from being detected as anindication of an arrhythmia of a particular type.

Morphological Stability Analysis

FIG. 21 is a block diagram illustrating an embodiment of a 1:1tachyarrhythmia classifier 2132. As a specific embodiment of 1:1tachyarrhythmia, classifier 432, 1:1 tachyarrhythmia classifier 2132includes a correlation input 2186, a majority voting circuit 2187, and amorphology stability analysis circuit 2188.

Correlation input 2186 receives a plurality of feature correlationcoefficient (Fcc) values each associated with an arrhythmic heart beatof a plurality of heart beats sensed during a 1:1 tachyarrhythmia. Eachfeature correlation coefficient (Fcc) value indicates whether theassociated arrhythmic heart beat is morphologically correlated to atemplate heart beat of a known type cardiac rhythm. Majority votingcircuit 2187 classifies the 1:1 tachyarrhythmia as a particular typetachyarrhythmia by a majority voting. That is, if the number of thearrhythmic heart beats that are correlated to the template heart beatequals or exceeds a predetermined threshold number, the 1:1tachyarrhythmia is classified as that particular type tachyarrhythmia.If the number of the arrhythmic heart beats that are correlated to thetemplate heart beat is smaller than the predetermined threshold number,morphology stability analysis circuit 2188 further classifies the 1:1tachyarrhythmia, based the stability of morphology as indicated by thefeature correlation coefficient (Fcc) values.

FIG. 22 is a block diagram illustrating an embodiment of a 1:1tachyarrhythmia classifier 2232. As a specific embodiment of 1:1tachyarrhythmia classifier 2132, 1:1 tachyarrhythmia classifier 2232includes correlation input 2186, a majority voting circuit 2287, and amorphology stability analysis circuit 2288.

Majority voting circuit 2287 is a specific embodiment of majority votingcircuit 2187 and includes a correlation comparator 2290, a correlatedbeat counter circuit 2291, and a first type tachyarrhythmiaclassification circuit 2292. Correlation comparator 2290 receives eachfeature correlation coefficient (Fcc) value from correlation input 2186and compares the feature correlation coefficient (Fcc) value to apredetermined detection threshold (Fcc_(th)). If the feature correlationcoefficient (Fcc) value is greater than the predetermined detectionthreshold (Fcc_(th)), correlation comparator 2290 indicates a correlatedbeat. Correlated beat counter circuit 2291 counts the number of thecorrelated beats (N_(CORR)) indicated by correlation comparator 2290.First type tachyarrhythmia classification circuit 2292 classifies the1:1 tachyarrhythmia as the first type tachyarrhythmia if the number ofthe correlated beats (N_(CORR)) equals or exceeds a predeterminedthreshold number. That is, the 1:1 tachyarrhythmia is classified as thefirst type tachyarrhythmia if a minimum number of the arrhythmic heartbeats out of the plurality of heart beats sensed during the 1:1tachyarrhythmia equals or exceeds a threshold number defining themajority.

Morphology stability analysis circuit 2288 is a specific embodiment ofmorphology stability analysis circuit 2188 and includes a median Fcccomputation circuit 2294, a spread Fcc counter circuit 2295, a secondtype tachyarrhythmia classification circuit 2296, and a third typetachyarrhythmia classification circuit 2297. Median Fcc computationcircuit 2294 computes a median feature correlation coefficient(Fcc_(MED)) being a median value of the plurality of feature correlationcoefficient (Fcc) values. Spread Fcc counter circuit 2295 counts afeature correlation coefficient spread number (Fcc_(SPRD)) being anumber of feature correlation coefficient (Fcc) values of the pluralityof feature correlation coefficient (Fcc) values that are within a windowcentered at the median feature correlation coefficient (Fcc_(MED)).Second type tachyarrhythmia classification circuit 2296 classifies the1:1 tachyarrhythmia as a second type tachyarrhythmia based on thefeature correlation coefficient spread number (Fcc_(SPRD)). If the 1:1tachyarrhythmia is not classified as the second type tachyarrhythmia,third type tachyarrhythmia classification circuit 2297 classifies the1:1 tachyarrhythmia as one of a third type tachyarrhythmia and the firsttype tachyarrhythmia based on the median feature correlation coefficient(Fcc_(MED)) and the number of the correlated beats (N_(CORR)).

In one embodiment, the template heart beat is a heart beat of an NSR.First type tachyarrhythmia classification circuit 2292 classifies the1:1 tachyarrhythmia as an SVT if the number of the correlated beats(N_(CORR)) equals or exceeds the predetermined threshold number. If thefeature correlation coefficient spread number (Fcc_(SPRD)) is smallerthan a predetermined threshold spread number, second typetachyarrhythmia classification circuit 2296 classifies the 1:1tachyarrhythmia as a polymorphic ventricular tachyarrhythmia (PVT). Ifthe feature correlation coefficient spread number (Fcc_(SPRD)) is notsmaller than the predetermined threshold spread number, the medianfeature correlation coefficient (Fcc_(MED)) is equal to or smaller thana predetermined threshold median, and the number of the correlated beats(N_(CORR)) is smaller than a predetermined threshold number, third typetachyarrhythmia classification circuit 2297 classifies the 1:1tachyarrhythmia as a monomorphic ventricular tachyarrhythmia (MVT). Ifthe feature correlation coefficient spread number (Fcc_(SPRD)) is notsmaller than the predetermined threshold spread number, the medianfeature correlation coefficient (Fcc_(MED), is greater than thepredetermined threshold median, and the number of the correlated beats(N_(CORR)) is equal to or greater than the predetermined thresholdnumber, third type tachyarrhythmia classification circuit 2297classifies the 1:1 tachyarrhythmia as an SVT.

FIG. 23 is a flow chart illustrating an embodiment of a method 2300 forclassifying a 1:1 tachyarrhythmia for the morphology-based 1:1tachyarrhythmia discrimination. In one embodiment, method 2300 isperformed by 1:1 tachyarrhythmia classifier 2132, including its specificembodiment, 1:1 tachyarrhythmia classifier 2232.

A plurality of feature correlation coefficient (Fcc) values are receivedat 2310. An exemplary distribution of such feature correlationcoefficient (Fcc) values are illustrated in FIG. 24. The plurality offeature correlation coefficient (Fcc values are associated with aplurality of arrhythmic heart beats sensed during a detected 1:1tachyarrhythmia. Each feature correlation coefficient (Fcc) valueindicates whether an arrhythmic heart beat is morphologically correlatedto a template heart beat of a known type cardiac rhythm. The number ofarrhythmic heart beats that are correlated to the template heart beat(N_(CORR)), i.e., the number of the arrhythmic heart beats associatedwith a feature correlation coefficient (Fcc) value that exceeds apredetermined threshold Fcc value (Fcc_(th)), is counted. If the numberof the correlated beats (N_(CORR)) equals or exceeds a predeterminedthreshold number (Fcc_(th)) at 2320, the 1:1 tachyarrhythmia isclassified as a first type tachyarrhythmia at 2330. If the number of thecorrelated beats (N_(CORR)) is smaller than the predetermined thresholdnumber, the stability of the feature correlation coefficient (Fcc)values are analyzed at 2340, and the 1:1 tachyarrhythmia is classifiedbased on the stability analysis at 2350.

The stability analysis looks into the distribution of the featurecorrelation coefficient (Fcc) values for the plurality of arrhythmicheart beats. A median feature correlation coefficient (Fcc_(MED)) iscalculated as the median value of the feature correlation coefficient(Fcc) values. A feature correlation coefficient spread number(Fcc_(SPRD)) is counted to indicate the number of feature correlationcoefficient (Fcc) values that are within a window centered at the medianfeature correlation coefficient (Fcc_(MED)). The 1:1 tachyarrhythmia isclassified as a second type tachyarrhythmia based on the featurecorrelation coefficient spread number (Fcc_(SPRD)). If the 1:1tachyarrhythmia is not classified as the second type tachyarrhythmia, itis classified as one of a third type tachyarrhythmia and the first typetachyarrhythmia based on the feature correlation coefficient spread andthe number of the correlated beats (N_(CORR)). In one embodiment, thetemplate heart beat represents a heart beat of an NSR, the first typetachyarrhythmia is SVT, the second type tachyarrhythmia is PVT, and thethird type tachyarrhythmia is MVT. The 1:1 tachyarrhythmia is classifiedas a PVT if the feature correlation coefficient spread number(Fcc_(SPRD)) is smaller than a predetermined threshold spread number. Ifthe feature correlation coefficient spread number (Fcc_(SPRD)) is notsmaller than the predetermined threshold spread number, the 1:1tachyarrhythmia is classified as an MVT if the median featurecorrelation coefficient (Fcc_(MED)) is equal to or smaller than apredetermined threshold median and the number of the correlated beats(N_(CORR)) is smaller than the predetermined threshold number, or as anSVT if the median feature correlation coefficient (Fcc_(MED)) is greaterthan the predetermined threshold median and the number of the correlatedbeats (N_(CORR)) is equal to or greater than the predetermined thresholdnumber.

FIG. 24 is a graph illustrating an exemplary distribution of featurecorrelation coefficient (Fcc) values 2400A-J for a plurality ofarrhythmic heart beats sensed during an episode of the 1:1tachyarrhythmia. The distribution of feature correlation coefficient(Fcc) values each associated with one of ten arrhythmic heart beats areshown. Four out of the ten feature correlation coefficient (Fcc) valuesare above the predetermined detection threshold (Fcc_(th)). That is, thenumber of the correlated beats (N_(CORR)) is equal to four. Because thenumber of the correlated beats (N_(CORR)) is smaller than the thresholdnumber (that defines majority) needed to classify the 1:1tachyarrhythmia as the first type tachyarrhythmia, a stability analysisfor the feature correlation coefficient (Fcc) values is performed. Thisincludes a calculation of the median feature correlation coefficient(Fcc_(MED)) and a counting of the feature correlation coefficient spreadnumber (Fcc_(SPRD)). The feature correlation coefficient spread number(Fcc_(SPRD)) is the number of Fcc values that fall into a window definedby Fcc_(MED)±δ(Fcc_(SPRD)=5 as illustrated in FIG. 24). Parametersincluding δ, the predetermined detection threshold (Fcc_(th)), thepredetermined threshold median, and the predetermined threshold numberare empirically determined. The number of the correlated beats(N_(CORR)), the median feature correlation coefficient (Fcc_(MED)), andthe feature correlation coefficient spread number (Fcc_(SPRD)), show thestability of the feature correlation coefficient (Fcc) values asmeasured by their distribution.

FIG. 25 is a flow chart illustrating a method 2500 for themorphology-based tachyarrhythmia discrimination. Method 2500 is aspecific embodiment of method 300 and includes an exemplary applicationof method 2300. Step 2570 in method 2500 is a specific embodiment ofstep 370 and includes method 2300 as discussed above.

While a known, simpler method classifies an 1:1 tachyarrhythmia as oneof SVT and VT by a majority voting based on the feature correlationcoefficient (Fcc) values, method 2500 provides for furtherdiscrimination between PVT and MVT as well as identification of“borderline” SVTs. This further eliminates unnecessary deliveries ofventricular defibrillation shocks. In one embodiment, a PVT isimmediately treated with one or more ventricular defibrillation shocks,while anti-tachycardia pacing (ATP) is delivered during on or moreattempts to terminate an MVT. Thus, when the majority voting does notresult in a classification of SVT, the stability analysis providesfurther bases for eliminating unnecessary ventricular defibrillationshocks that cause significant discomfort to the patient and shortens thelife expectancy of an ICD.

In General

Classification of a 1:1 tachyarrhythmia can be performed usingvariations of the embodiments and/or various combinations of theembodiments discussed above without deviating from the concepts embeddedin these embodiments. In one exemplary combination, a system performingmethod 300 includes a sub-system for performing method 700 as step 330and another sub-system for performing method 2300 as step 370. Inanother exemplary combination, a system performing method 300 mayinclude a sub-system for performing method 1800 as step 340 and anothersub-system for performing method 2300 as step 370. In another exemplarycombination, a system performing method 300 includes a first sub-systemfor performing method 700 as step 330, a second sub-system forperforming method 1200 as step 340, and a third sub-system forperforming method 2300 as step 370. Other possible combination of theembodiments discussed above will be apparent to those skilled in theart, upon reading and comprehending this document.

The relationship between a rate and an interval, as used in thisdocument, is the relationship between a frequency and its correspondingperiod. If a rate is given in beats per minute (bpm), its correspondinginterval in millisecond is calculated by dividing 60,000 by the rate(where 60,000 is the number of milliseconds in a minute). Any process,such as a comparison, using the rates is to be modified accordingly whenthe intervals are used instead. For example, if a tachyarrhythmia isdetected when the ventricular rate exceeds a tachyarrhythmia thresholdrate, an equivalent process is to detect the tachyarrhythmia when theventricular interval falls below a tachyarrhythmia threshold interval.The appended claims should be construed to cover such variations. Forexample, atrial and ventricular intervals should be construed asequivalent to the atrial and ventricular rates, respectively.

It is to be understood that the above detailed description is intendedto be illustrative, and not restrictive. For example, system 120,circuit 225, and their various embodiments as discussed in this documentare not limited to applications in an ICD, but may be incorporated intoany arrhythmia analysis system, such as a computer program for analyzingpre-collected cardiac data. Other embodiments will be apparent to thoseof skill in the art upon reading and understanding the abovedescription. The scope of the invention should, therefore, be determinedwith reference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

1. A system for classifying tachyarrhythmias, the system comprising: atemplate generation circuit configured to produce a template waveformassociated with a template heart beat of a known type cardiac rhythm; atemplate waveform storage circuit coupled to the template generationcircuit, the template waveform storage circuit configured to store thetemplate waveform; a feature locating circuit configured to select aplurality of arrhythmic morphological features on an arrhythmic waveformassociated with an arrhythmic heart beat of a tachyarrhythmia andproduce timing information indicative of locations of the plurality ofarrhythmic morphological features on the arrhythmic waveform; and afeature extracting circuit coupled to the template waveform storagecircuit and the feature locating circuit, the feature extracting circuitconfigured to locate a plurality of template morphological features onthe stored template waveform using the timing information.
 2. The systemof claim 1, wherein the template generation circuit comprises: atemplate waveform alignment circuit configured to align a plurality ofwaveforms each associated with a heart beat of the known type cardiacrhythm by a fiducial point; and a template averaging circuit coupled tothe template waveform alignment circuit, the template averaging circuitconfigured to produce the template waveform by averaging amplitudes ofthe plurality of waveforms.
 3. The system of claim 2, wherein thetemplate waveform alignment circuit comprises a peak detector configuredto detect a peak of a depolarization, and wherein the fiducial point isthe peak of the depolarization.
 4. The system of claim 1, wherein thefeature locating circuit comprises: a waveform input configured toreceive the arrhythmic waveform; a fiducial point locating circuitcoupled to the waveform input, the fiducial point locating circuitconfigured to detect a fiducial point on the arrhythmic waveform; afeature selecting circuit coupled to the fiducial point locatingcircuit, the feature selecting circuit configured to select theplurality of arrhythmic morphological features on the arrhythmicwaveform based on one or more predetermined criteria; and a featuretiming circuit coupled to the feature selecting circuit, the featuretiming circuit configured to measure time intervals each between thefiducial point on the arrhythmic waveform and one arrhythmicmorphological feature of the selected plurality of arrhythmicmorphological features.
 5. The system of claim 4, wherein the fiducialpoint locating circuit comprises a peak detector configured to detect apeak of a depolarization on the arrhythmic waveform.
 6. The system ofclaim 4, wherein the feature extracting circuit is configured to locatethe plurality of template morphological features on the templatewaveform based on the fiducial point on the arrhythmic waveform and themeasured time intervals.
 7. The system of claim 6, wherein the featureextracting circuit is configured to locate a corresponding fiducialpoint on the template waveform, align the fiducial point on thearrhythmic waveform and the corresponding fiducial point on the templatewaveform, and locate the plurality of template morphological features onthe template waveform using the measured time intervals each as a timeinterval between one template morphological feature of the plurality oftemplate morphological features and the fiducial point on the templatewaveform.
 8. The system of claim 1, further comprising a correlationanalyzer configured to produce a correlation coefficient indicative of acorrelation between the selected plurality of arrhythmic morphologicalfeatures and the located plurality of template morphological features.9. The system of claim 8, wherein the tachyarrhythmia comprises a 1:1tachyarrhythmia, and further comprising a 1:1 tachyarrhythmia classifierconfigured to classify the arrhythmic heart beat using the correlationcoefficient and a predetermined correlation threshold.
 10. The system ofclaim 9, wherein the 1:1 tachyarrhythmia classifier is configured toclassify the arrhythmic heart beat as about of a first typetachyarrhythmia response to the correlation coefficient being greaterthan the predetermined correlation threshold and a beat of a second typetachyarrhythmia in response the correlation coefficient not beinggreater than the predetermined correlation threshold.
 11. A method forclassifying tachyarrhythmias, the method comprising: producing atemplate waveform associated with a template heart beat; storing thetemplate waveform; receiving an arrhythmic waveform associated with anarrhythmic heart beat; selecting a plurality of arrhythmic morphologicalfeatures on the arrhythmic waveform; producing timing informationindicative of locations of the plurality of arrhythmic morphologicalfeatures on the arrhythmic waveform; and locating a plurality oftemplate morphological features on the template waveform using thetiming information.
 12. The method of claim 11, wherein receiving thearrhythmic waveform comprises receiving a waveform associated with aheart beat sensed during a 1:1 tachyarrhythmia.
 13. The method of claim12, further comprising: producing a correlation coefficient indicativeof a correlation between the selected plurality of arrhythmicmorphological features and the located plurality of templatemorphological features; and classifying the arrhythmic heart beat usingthe correlation coefficient and a predetermined correlation threshold.14. The method of claim 13, further comprising classifying thearrhythmic heart beat as a beat of a first type tachyarrhythmia inresponse to the correlation coefficient being greater than thepredetermined correlation threshold and a beat of a second typetachyarrhythmia in response to the correlation coefficient not beinggreater than the predetermined correlation threshold.
 15. The method ofclaim 14, wherein the template heart beat is a heart beat sensed duringa normal sinus rhythm, the first type tachyarrhythmia is asupraventricular tachyarrhythmia, and the second type tachyarrhythmia isa ventricular tachyarrhythmia.
 16. The method of claim 13, whereinproducing the template waveform comprises: aligning a plurality ofwaveforms each representing a heart beat of a known type cardiac rhythmby a fiducial point; and producing the template waveform by averagingamplitudes of the plurality of waveforms each representing the heartbeat of the known type cardiac rhythm.
 17. The method of claim 13,wherein producing the timing information indicative of the locations ofthe plurality of arrhythmic morphological features on the arrhythmicwaveform comprises: detecting a fiducial point on the arrhythmicwaveform; and measuring time intervals each between the fiducial pointand one arrhythmic morphological feature of the selected plurality ofarrhythmic morphological features on the arrhythmic waveform.
 18. Themethod of claim 17, wherein detecting the fiducial point on thearrhythmic waveform comprises detecting a peak of a depolarization onthe arrhythmic waveform.
 19. The method of claim 17, wherein locatingthe plurality of template morphological features on the templatewaveform comprises locating the plurality of template morphologicalfeatures on the template waveform based on the fiducial point on thearrhythmic waveform and the measured time intervals.
 20. The method ofclaim 19, wherein locating the plurality of template morphologicalfeatures on the template waveform comprises: locating a correspondingfiducial point on the template waveform; aligning the fiducial point onthe arrhythmic waveform and the corresponding fiducial point on thetemplate waveform; and locating the plurality of template morphologicalfeatures on the template waveform using the measured time intervals eachas a time interval between one template morphological feature of theplurality of template morphological features and the fiducial point onthe template waveform.